Greater North Sea key signals

Ecosytem and environmental trends

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​​​​​​​​​​​​​​The North Sea is characterized by episodic changes in the productivity of key components of the ecosystem. Phytoplankton, zooplankton, and demersal and pelagic fish have all exhibited such cycles in variability. Managers should expect change and ensure that management plans have the potential to respond to new circumstances. Examples of these changes include the gadoid ‘outburst’ in the 1970s, and a reduced productivity of herring Clupea harengus since 2002.​​​

The changes have been described as regime shifts; a notable example is the composition of the zooplankton community, which changed both in terms of species and size composition in the late 1980s and again around 2000.

The temperature trends of the North Sea are linked to these ecosystem changes. Whilst the mechanisms underlying this link are not known, it is clear that the temperature cycle of the North Atlantic (the Atlantic Multidecadal Oscillation, AMO) affects the North Sea. Examples of this include the outbursts of anchovy Engraulis encrasicolus ​​in the 1930s and the 2000s.

Recent years have seen an increase in the addition of new artificial hard substrate to the North Sea (platforms, wind turbines, and rocks). These additions have changed the biodiversity and productivity in local areas.

Fishing has reduced the number of large fish in the North Sea ecosystem (mostly cod Gadus morhua,​ saithe Pollachius
virens​, ling Molva molva​, sturgeon Acipenser sturio​​, and some elasmobranchs). In historical times, the large whale populations of the North Sea were depleted or extirpated by hunting. Whilst the impact of these removals on the ecosystem functioning is not clearly understood, it should be assumed that the North Sea ecosystem is currently in a perturbed state. Several of these elasmobranch species are now considered threatened or endangered by OSPAR and IUCN and are still caught as bycatch in fisheries.

However, it is clear that fishing effort has reduced in the North Sea since the 2002 CFP reforms; this can now be detected in the reduction of fishing mortality in most assessed fish stocks and an increase in the amount of larger fish present. The majority of assessed fish stocks are now fished at or below MSY fishing mortality targets (FMSY).

There have been reports of a shift from pelagic to benthic ​production. The plaice stock in the North Sea has increased almost fourfold in the last 15 years and is now larger than when monitoring began in the 1950s. Flatfish are not included in the current multispecies models for the North Sea. The consequences of this large stock for the ecosystem are unknown.

The seabird population showed an overall increasing trend until 2000, after which it declined. Recent changes in fisheries management policy (e.g. reduction in effort and the landing obligation) will likely affect seabirds as well as other parts of the ecosystem.

There have been no significant changes in the cetacean populations of the North Sea. The centre of distribution of the harbour porpoise Phocoena phocoena population moved southwards from off the east of Scotland to the southern North Sea between the mid-1990s and the mid-2000s. These animals are caught as bycatch in bottom-set gillnets. The abundance of grey seals has been increasing for at least 30 years.